ORLANDO, USA - JUNE 13: Pictures of one of the massacre victims left at a make shift memorial at Phillips Center for the Performing Arts in Orlando, USA on June 13, 2016. (Photo by Samuel Corum/Anadolu Agency/Getty Images)

ORLANDO, USA - JUNE 13: Pictures of one of the massacre victims left at a make shift memorial at Phillips Center for the Performing Arts in Orlando, USA on June 13, 2016. (Photo by Samuel Corum/Anadolu Agency/Getty Images)

People wave rainbow flags and hold portraits on the Place de la Comedie in Montpellier on June 13, 2016 as they pay homage to the victims of a shooting at a gay nighclub in Orlando. / AFP / SYLVAIN THOMAS (Photo credit should read SYLVAIN THOMAS/AFP/Getty Images)

Since he was a boy, Anthony Luis Laureano Disla loved to dance. Early Sunday morning, he was dancing and laughing with his friends at Pulse nightclub in Orlando when a gunman stormed and shot him. Laureano Disla was 25. His obituary is linked in our bio. #PulseShooting #OrlandoUnited #PrayForOrlando #OrlandoStrong

Luis Daniel Wilson-Leon moved from Puerto Rico to Vero Beach, longtime friend Daniel Gmys-Casiano said, and was immediately promoted to manager at a shoe store. He was a protector, confidant and hero, Gmys-Casiano said. Wilson-Leon died in the Pulse nightclub shooting in Orlando. He was 37. His obituary is linked in our bio. #PulseShooting #OrlandoUnited #PrayForOrlando #OrlandoStrong

Enrique L. Rios left his home in New York to spend the weekend celebrating a friend's birthday in Orlando. He was killed in the mass shooting at Pulse nightclub Sunday. He was 25. His obituary is linked in our bio. #PulseShooting #OrlandoUnited #PrayForOrlando #OrlandoStrong

To Jean Carlos Mendez Perez, good living was all about looking, smelling and feeling his best, loved ones said. He and his longtime partner, Luis Daniel Wilson-Leon, were together at Pulse nightclub early Sunday, and both were among the victims who died in the shooting. Perez was 35. His obituary is linked in our bio. #PulseShooting #OrlandoUnited #PrayForOrlando #OrlandoStrong

In a statement cited by ABC News on Tuesday, Clinton's campaign said: "The rally was a 3,000-person, open-door event for the public. This individual wasn't invited as a guest and the campaign was unaware of his attendance until after the event."

Clinton's press secretary, Nick Merrill, followed up that statement with another, saying Clinton "disagrees with [Mateen's] views and disavows his support."

RELATED: Clinton, Trump on key issues regarding Orlando shooting

Mateen has a history of making controversial statements, including an assertion that the shooting that his son carried out at the gay nightclub Pulse was influenced by homophobia. "God created man for woman," Mateen said on Facebook days after the massacre.

His presence at the rally raised questions about background checks for people in attendance. ABC News cited an unnamed Secret Service official who said people at the rally "all received the exact same screening."